This Merlin teed up at Tiscornia a couple days ago. It was noticeably pale.
I couldn't remember in the field what the criteria where for the plains subspecies, but knew it had to do with the spread tail and the wing patterning.
As it turns out (per Wheeler's Raptors of Eastern North America) "Boreal" Merlin (the one we usually get around here) has 3 or 4 pale tail bands, "Prairie" has 4 or 5...
The other big mark is that in the underwing the pale spots cover more than 50% of the feathers in "Prairie:"
As a comparison here's a more typical bird from Tiscornia some time in the past:
If accepted this will be a 2nd state record (though I need to dig up another one we had a few years ago in the spring, and I've seen Tim's pics of another bird in the south county), so they're probably more common than people realize, but are really hard to document unless you have birds frequently teeing up the way they do at the beach.